The present invention relates to a "Heads-Up Display" sighting system, called a HUD for use on shotguns, rifles and pistols, preferably for the shotgun target market such as skeet, sporting clays, and trap. A HUD and reflex sight are similar products in that the scene is not reimaged by the optics and that the sight magnification is 1X unity.
In target shooting such as skeet and the like, the moving target is not a target for very long. The shooter must react to his initial vision of the target, bring the gun to a sighting position, locate the target in the sight, fine-tune the aim of the gun on the target through the sight, and fire. These activities must take place in time frames of a second or less. One of the most critical of these activities is target acquisition within the sight. That is, it is not enough merely for the shooter to see the target and "shoot from the hip." Reliable accuracy requires actually finding the target within the gunsight or scope. This is called "target acquisition."
Thus, it is important to minimize the time or maximize the speed with which the target can be acquired within the sight. The quicker this can be done, the quicker the later steps of actually refining the aim and pulling the trigger can be accomplished.
The number of reflex sight models (1X unity power) available in the market has doubled since 1992. As a general trend, the newer models have larger aperture diameters to accommodate less critical eye alignment and give a larger scene field of view. The early models were packaged in 1 inch and 30 mm tubes, which required scope rings for mounting. Newer models with larger tubes and tubeless designs usually have a mounting system integral to the sight. The early model aimpoints were always red dots of 3 to 6 arc minutes in diameter. Since the competition handgun market is responsible for most of the current development with reflex sights, aimpoint sizes and features have been optimized for handguns and their targets.
The HUD or reflex sight is made up of an optical collimating reflector, mechanical adjustments and packaging, and an electronic light source. Conventional optical methods for collimating and reflecting the aimpoint to the eye use very basic classical optics. The reflex sights are one or two element off-axis reflectors with cover windows to zero the optical power of the scene (near unity 1X magnification) and/or provide environmental seals. To combine the aimpoint wavefront with the scene, the typical reflex sight uses a partially mirrored coating or, for more efficiency, a multilayer dielectric dichroic coating, which reflects only the deep red aimpoint and transmits the visible spectrum of the scene. The hologram relies on diffraction to bring the red aimpoint into the scene.
Glass is the optical material of choice for most sights. Reflex designs are simple reflectors which have a small optical element volume. For this reason the durability and optical properties of glass offset the potential benefits of plastic. Plastic is light weight, moldable for easy aspheric surface production, and cost effective. But plastic's thermal stability, durability, and optical properties are inferior to glass. Other than the holographic sight, reflex optical designs have been traditional and not employed the benefits of aspheres, gradient index glass, and the many types of diffractive optics. Cost versus the design performance advantages usually controls these variables.
The adjustments for aligning the aimpoint axis to the firearm for windage and elevation are usually implemented by precision mechanics, such as are shown in U.S. Pat. No. 5,369,888 to Kay et al., the entire disclosure of which is incorporated herein by reference. A reflex sight design can change the point of impact by adjusting any one of the following, tilting the reflector, decentering the reflector, or decentering the aimpoint source. For the hologram sight only tilt of the holographic window steers the aimpoint. A tilt of the entire assembly is common to all sights. Aluminum is the standard packaging material for most sights. Recently, there have been a few products that have used composites and plastics to reduce weight and cost. Some of these materials have less dimensional stability than aluminum and require the packaging design to be more thorough for collimation and alignment retention over the operating conditions.
The light emitting diode, LED, is the most common light source used in the battery powered sights for its power efficiency and high brightness properties. The typical red dot aimpoint is created by the LED projecting light over a fan angle through a pinhole in an opaque material such as metal or coated glass. The pinhole has a specific size and uniformity so when magnified by the collimating optics, it has the desired angular subtense to overlay with the see-through scene. The aimpoint can be more than a simple dot. Complex reticles can be photo etched onto a glass substrate and can have different gray levels. The only restriction is that larger reticles require the optical design aberrations to be corrected over the field of view of the reticle. Most sights on the market use deep red 670 nm wavelength LED's. The reasons for this are that red LEDs are usually the brightest, red has good color contrast with a green scene, and that the optical reflector coatings can efficiently reflect red without disturbing the transmission efficiency of the scene since the 670 nm LED is near the edge of the visible spectrum (400-700 nm). The holographic sight uses a 670 nm diode laser as a high brightness monochromatic source to illuminate the hologram. The battery sources are typically lithium, silver oxide, and alkaline. Aimpoint brightness is controlled by 10 to 15 position variable resistors or rheostats that usually reduce the brightness by a factor of two between positions.
Holographic sights have the greatest advantage for target acquisition, because they have open apertures which can be located closer to the eye than other sights. The hologram design permits the aimpoint light source to illuminate the holographic window from the front so the light source does not have to be between the combiner and the eye. As a result, the entire sight can be moved closer to the eye within 100 mm (4 inch) or to the minimum safe eye distance.
There is a relationship between the size of the aperture and the focal length of the reflector, which is roughly the distance to the LED point source. The name for this optical parameter is F-number (F#), which is the focal length divided by the aperture diameter in equivalent units. As the F# gets below three, the relative power on the optics increases, so that simple spherical surfaces can cause noticeable amounts of spherical aberration. If the eye pupil were as large as the entire collimation aperture, then the aimpoint would appear to have a halo blur. Since the eye pupil is typically much smaller than the collimation aperture, the aimpoint appears in sharp focus. But, as the eye decenters in the collimation aperture, the spherical aberration causes an angular deviation to the aimpoint, which is perceived to the eye as parallax.
Examples of prior sights based on the holographic design are U.S. Pat. No. 4,730,912 to Loy et al. and U.S. Pat. No. 5,483,362 to Tai et al. Non-holographic reflex sights are seen in U.S. Pat. No. 4,665,622 to Idan and U.S. Pat. No. 5,373,644 to DePaoli. Both holographic and conventional reflex sights have their limitations.
FIG. 4 is a raytrace layout of a 25.4 mm aperture off-axis F/3.0 reflex sight. Note that the reflector is used off-axis to keep the aimpoint source from obscuring the collimation aperture to the eye. The F/3.0 off-axis reflector has similar adverse aberration properties as an F/1.5 on-axis reflector. The parallax correction of this design is +/-0.1' arc minute on-axis and +/-1.3' off-axis at 0.5 degree.
Enlarging the aperture to 40 mm with the same focal length produces an F/1.9 reflector with a parallax correction of +/-1' on-axis and +/-3' off-axis at 0.5 degree. Enlarging the aperture to 50 mm with the same focal length produces an F/1.5 reflector with a parallax correction of +/-4' on-axis and +/-5 off-axis at 0.5 degree. All of these designs have spherical surfaces, and it seems that the F/1.9 off-axis reflector is the limit for acceptable performance. Note that the distortion of the see-through scene is not quantified, but it will increase as the reflector F/# is reduced. An aspheric off-axis F/1.9 reflector design has the benefit that the non-reflecting surface of the lens can remain flat, so there should be minimal see-through distortion. The F/1.9 aspheric design parallax correction is only slightly better than the F/1.9 spherical design.
The geometrical layout of a tubeless reflex sight provides a good or the required field of view for target acquisition by using an F/1.9 off-axis 40 mm reflector located 228 mm from the eye with the aimpoint source and packaging extending towards the eye 110 mm, which still leaves 100+ mm of mechanical eye relief. This concept works but it can never obtain a super wide field of view. A classical aircraft HUD has the freedom to locate the collimating optics below the dashboard so the aimpoint is combined by a plate beam splitter which has minimal distortion. This arrangement on a firearm almost always leads to an optical sight axis elevated too high above the barrel axis for practical use.
Thus, there remains a need in the art for an improved gunsight with a superwide field of view, good registration of aimpoint with target, minimum aberration parallax, and minimum obstruction of scene view.